1. Field of the Invention
This invention relates to the recovery of oil from hydrocarbon reservoirs, and particularly relates to in-situ recovery of heavy hydrocarbons such as kerogen from oil shale and residual hydrocarbon from conventional oil wells after primary recovery.
2. Description of Related Art
Many hydrocarbon bearing formations do not flow hydrocarbons freely to the wellbore for extraction because of the high viscosity and/or solid state of the hydrocarbons. For example, kerogen in an oil shale is a high molecular weight hydrocarbon requiring temperatures over 300 degrees C. before it will break down and separate from the formation rock. In conventional oil wells, the primary recovery of hydrocarbons varies considerably, but typically about 30% of the hydrocarbons will be removed after the well stops producing economically. The remaining hydrocarbons are higher viscosity and/or higher molecular weight components of the original hydrocarbons, that will not flow into the wellbore for recovery after the primary oil recovery. In some conventional oil wells, a significant fraction including all of the oil may be heavy oil that will not flow freely to the wellbore without temperature and/or chemical intervention. In tar sands, the naturally occurring hydrocarbons do not flow freely to a wellbore.
For oil shales, current technologies include freezing pockets of the formation, and heating the formation within each pocket to recover kerogen from the formation. Such processes are energy intensive and require the drilling of multiple wells to recover kerogen from a relatively small section of the formation. An alternate oil shale process includes circulating heated combustion gas in a formation, but these processes introduce carbon dioxide into the formation that must be separated from any produced fluids, and are designed to work in water-free environments.
Oil shales and tar sands may also be recovered through bulk strip mining. The bulk material is mined out of the ground, and various surface processes can be utilized to strip any hydrocarbons from the bulk. Other mining techniques are possible, and such techniques inherently leave more of the hydrocarbons unrecovered than strip mining. Any of the mining processes introduce a number of environmental issues, including disposal of solvents, recovery of the mined land, and disposal of the shale remainder after the bulk of the hydrocarbons are removed.
For secondary recovery of oil wells and for oil wells with inherently heavy oil, several processes are available in the current technology. Some wells may be flushed with viscous fluids such as polymer based gels that rinse remaining oil from an injection well to an extraction well. The flushing process is expensive because of the fluid costs, and can only recover fluids that are essentially low viscosity although perhaps a bit higher viscosity than the oil recovered in the primary recovery. The flushing process is also subject to channeling between wells which can prevent full recovery of oil; channeling can be mitigated with fluid loss additives but these introduce damage into the formation. Further, some formations are sensitive to the introduction of water (e.g. formations with a high clay content) and therefore the flushing process is either ineffective or requires expensive anti-swelling additives to the fluid.
Secondary oil recovery has also been attempted with low-grade burning in the formation. The flame front in the formation reduces the viscosity of the remaining oil and drives the oil to an extraction well. The flame recovery process is difficult to initiate and control, it inherently consumes some of the oil in the formation, and it introduces combustion byproducts into the final produced fluids.
The processes in the current technology produce final products that have high molecular weight hydrocarbons. Low to middle weight hydrocarbon products (e.g. five to twelve carbons per molecule) are inherently more commercially valuable than heavy hydrocarbons. Some processes use a portion of the recovered hydrocarbons in the extraction process, for example burning them to heat some aspect of the recovery system. Further, as the recovery process proceeds, the molecular composition of the produced gas changes, often with lighter molecules recovered earlier and heavier molecules recovered later. Whether the produced fluids are burned or utilized as a product for sale, the changing of the molecular composition of the produced fluids introduces complications that must be managed.